Frequency control system



' Aug. 22, 1944. Ayc;- TRAvls I 2,356,483`

FREQUENCY ONTROL SYSTEM Filed July 2, 1942 INPUT Patented Aug. 22, 1944 FREQUENCY CONTROL SYSTEM Charles Travis,-deceased, late of Wyndmoor, Pa.,

by Yvonne Travis, administratrix,.Wyndmoon Pa., assigner to Philco Radio jand `Television Corporation, Philadelphia, Pa., a corporation of Delaware Application'July 2, 1942, Serial No.l V449,505V 17 Claims. (Cl. 25o- 36) This invention relates to frequency control systems, and more particularly to circuits for obtaining quadrature voltages in frequency control systems. l 'A There are now various usages of controllable oscillator circuitsin which it is desired to change the frequency by applying a control voltage. One of the most useful applications of these circuits is in frequency modulation transmitters where it is desired to frequency-modulate a carrier Wave in accordance wtih an audio frequency signal or thelike. Another use of such a circuit is in alternate carrier television, where one frequency control system may be used to maintain the normal frequency ofthe oscillator at a xed value, and a second may be used to shift the oscillatorto change the carrier frequency at periodic intervals. Other u ses of automatic frequency control are known in the electronics art and will not be enumerated in full here.

It is customary, in a frequency control circuit,V to employ at least one control tube which is caused to simulate a controllable reactance by means of a so-called quadrature voltage which is in phase quadrature relation with the oscillator tank circuit voltage. It is important that the phase of the quadrature voltage be accurate and that its magnitude be sufficient in all instances. Control circuits employed in the past have been deficient in these respects, particularly at high frequencies, e. g. above 2 megacycles.

It is one object of the present invention to produce accurate quadrature phase voltage in an oscillator frequency control circuit.

It is a further object of the invention to increase the magnitude of the quadrature voltage which may be derived 'from a given signal source.

It is another object of the invention toprovide a phase quadrature circuit in which the capacitance of the control tubes do not affect the accurate operation of the oscillator.

It is another object of the invention tof provide a frequency control system for an oscillator fhaving substantially linear control of the'frequenc'y by the control voltage.

Other objects of the invention will be apparent from the followingr description and the accompanying drawing, in which:

Fig. l is a wiring diagram of the preferred embodiment of the invention employing a balanced oscillator and a balanced frequency control sys-V of the invention usingV a' single direct current control voltage;

Fig. 3 is a wiring diagram of another modified formA of the invention inwhich the anodes of the control tubes are connected to thev same point on the oscillator tank circuit inductance; and

Fig. 4 is a'wiring diagram of further modification.

`Referring rst to Fig..1, there is shown an oscillator comprising tubes I and 2, a plater coil 3 and a grid coil 4. The plates 5 and 6 are connected to the opposite ends of they plate tank coil 3, and the center tap 1 of the plate coil is connected to a source of high voltage plate potential. The tank coil 3 may be tuned .by the distributed capacity of the winding, or if desired a tuning condenserk may be connected thereacross. The grids 8A and 9 may be connected'to the ends of the grid coil 4, the center tap ID 'of which may be connected to ground through a suitable biasing resistor I I by-passed by the condenser I2. Also coupled to the plate coil '3 is the output coil I3 which is connected to a load.v

e. g.. a further amplifier or an antenna system,V or, in the case of a radio receiver the first detector or converter. V'Also shown connected to the ends of the platecoil 3 are the plates I4 and I5 of the frequency control vacuum tubes I 6 and Il, which are shown in the drawing las triodes,

but which may be any other form of multi-element tube as is well known in the art. y

` Connected across the tank coil 3 is a phasingcircuit which includes the serially connected resistor I8 and condenser i9. This circuit also includes a resistor 20 and a lcondenser 2 I, in parallel, connected between the junction point 22V (of elements I8 'and I9), and the cathodes 23-24. This connection may also include the direct current blocking condenser 25. A s will be described later, the impedances of elements I8, I9, 20 and 2I are related in a particular manner so that a substantially perfect quadrature excitation volt- -age may be obtained;Y therefrom and applied to the grids 26 and 21 which areconnected for radio i frequency to the junction point 22 through direct current blocking condensers 28 and 29, and the direct current blocking4 condenser 25. C'onnected between the cathodes 23 and 24 and ground is a conventional biasing resistor 30 and its Icy-passv condenser 3,I.`Since thev radio frequency by-pass condenser 3|, the direct current blocking condenser 25, and the plate supply by- Dass condenser C all have negligible impedance at the oscillator frequency, the parallel elements 2li-2l of the-quadrature voltage deriving cir- The circuit shown in Fig. 1 is particularly adapted for operation from a pair of control voltages,7 and these voltages are preferably equal and opposite and balanced to ground. Complete cir'-H From this equation it is apparent that when the absolute values of R and :l: are equal, the ratio will be a pure imaginary,v indicating that the voltage e appliedto the grids-26 and 21 is exactly 9 0f.o.ut of phase with the `voltage E or E appearing at the two ends of the tank coil. This is, of

course, the desired condition.

Y .From Vthe foregoing relations, it may also be cuits for the obtaining of these control voltages.'V

are shown in United States P atent'qNo. 2,240,428, .patented April 29, 1941, and in the copending ap;`

plication of David B. Smith, Serial No. 401,494, led July 8, 1941.

As is well known, in a system of this general type the quadrature voltage causes the control tubes to appear as reactances which are variable' by the control signal to vary theY Wfrequencyxof the oscillator. The present invention Vis concerned particularly with Vthe means '.ior' obtaining the quadratureA voltage, as will now 4be, described in detail;v .iv

In order to derive'the fullbenet of the present invention, the resistor v2l) should be made equalfto the resistor I8, and condenser 2| should have the same reactance as condenser I9. -gWitn these preliminary conditions fuliilled, the currentli in-the circuit which includes plate-tocathode of the tube l1, the parallel circuit 2li-2|, thecondenser and the condenser [9 may be represente ed by the equation.' A f- ,'I1,=E 'w where E is the voltage-appliedto; the plate IS'by the oscillator tank coil, e is -thevoltage to ground at the junction 22, and-:c is the reactance of the condenser i9, which has already been set equal-to the reactance of the condenser 2l. The current I2 through ,the circuit which includes plate-tocathode of the tube `I 6, the parallel circuit 20-21, the condenser 25, and the resistor II8 may be represented by the equation in which R is the resistance oi resistor I 8. In the above equations, the voltages on the'plate's of the. d

Wherey l -l 1 l A Substituting 'the aforementioned values of I1 andfIz and rearranging-terms gives'the equation v2e 'x R seen that the quadrature excitation voltage, when the absolute values of R and :I: are equal, is equal `toA one-fourth theend-to-end tank voltage. This is ea substantially higher voltage than can be obtained from the conventional series resistancecapacitanlce circuit.

Referring now to Fig. 2, there is shown a modified form of the control circuit in which a single control voltage is used and quadrature radio frequency grid voltages are yapplied to grids 29a. and 21a in opposite phase. `The oscillator circuit may be the saine'as shown in Fig..1, or it may take any other suitable form. For economy of space', only, the oscillator tank coil is shown. IIL-this instance the grids of the frequency control tubes are supplied with quadrature voltages through a phase shifting network system comprising the resistors 3E and 31, the lcondensers 38 and 39,.and the parallel networks 40 and 4|. It is to be noted that the resistance-capacitance network comprising theresistance 3S and capacitance 38 is reversed relativerto the network comprising the resistance 31.and`the capacitance 39, so that the phase on grid 26a is 90 advanced while the-phase on gridf21a is 90 retarded, thereby placing both grids in phase vquadrature with the oscillator tank circuitbut out Iofphase with eachother. A single ,alternating current (e. g. audio) `or. direct current'control voltage may then be used to control the oscillator frequency. In the circuit shown in Fig. 2` the parallel circuits 40 and 4I correspond to the parallel circuit in Fig. 1 and coact with the elements 36, 38,V and 31, 39, respec tively, in the same manner as described in connection withrFig. ,1. The relation obtains in this Yinstance for the same reasons pac-itance circuitsj 42-43 and 44-45, .respec tively. AThe parallel networks 46 and 41 areconnected between the junction points of the series networksgand the `cathodes of the control tubes throughY ground. In this case, as in the form of invention shown in Fig.v 1, two control voltages of opposite phase are required. In this instance also the relation obtains Vfor the reasons previously Shown.

Reierring 'generally to the above-describedl em- .'showntheretarethree diierentvoltages 'involved, i.=,e..the control No1tage,thequadrature voltage, 'and the outputfvcltageforthe control-itunes. .It is Vages .must be appliedfin-oppositephase while the thirdmust applied in'likephase. Thus in Fig. l, `for example, fthe @control 'voltage iis applied :to .the grids :of :thefr'control tubes' 'in-npposite phase;

ythe `.output Noltages Aof fthe 'control :tubes are :applied to the ftank'r'coil tinopposite zplia'se;r and the quadrature vnltage tis applied' Ito the fgrids' offfthe control itubesfinfalikeiphase. .sFigsL f1 to .3 of the drawing; show ithe three possible arrangements of :the balanced system and :illustratelthe l ipreserrt .invention :to reach :iar-

application of fthe rangement.

lThe invention iis mot limited, however, to .the use .-.of balanced 'orrpush-pull:icontroltubes Thus in Fig. 4 there :is shown anembodiment:ofizhe-injL vention employing sa .single -fco'ntrol .tub'e 11.61: and :s a single oscillator tube lc having iranassociatedv tank vcircuit 53o; rIn athis instance ithe #plate vcir#r cuit .of the control `tube fis iinductively .coupled to the `tank circuit. v.The eseries:resistance-capacitance oircuit4849 is=connected .across 'the 'tank circuit; :The parallel metwork .50 Lis connectedbetween the `junction fpcint fof the :series :circuit and `:the fcathode -oT the fc'ontrol tube through ground. The quadrature -volta'ge :is lapplied fto .It will be understoodrof course, -tlriattiieinven-r tion ,is not limited ,to the .specie embodiments shown. .For example, the tubes may be multigrid tubes instead of -the simple ltriodes illustrated; -and other .modifications imay Ybe vmade without departing from .the invention.- What is lclaimed :is: 1

the .condor-:sers are ee'qua'l, fand' .1. .A frequency .control `circuit for Vshifting 'the frequency :of fan .oscillator having faftank coil,'/ comprising a Ipair `of rcontrol :tubes having triode elements,lmeans connecting the ,plate of each of,

said tubes 'to the oscillator tank coil, -a network*- including series-,connected resistance Afand :capacitance elements connected =in.- shunt to said tank coil, means r connecting the-grids xof gzsaid rtubes 'to the junction of Asaid elements, a com'iection *.Ibe-

-tween the licathodes of said atubeswand said junction, :and a network including 4roar.allel-tcormec'ted. resistance fand-capacitance xelements in saidcconnection, .said irstfnamedV resistance :and:capaci-` tance elements =befingzequal v^respecti-ve'ly towsaid last-named :resistance and Icapacitanoo"elements, the resistance 'of @eachY of lsaid rsistance lements being `substant'ially :equal fto ztlie reactance of 'each of said capacitance elements "at 'the .fop-

eratingirequency. v

.2.' In combination, an electrontub'e oscillator capableof 'having' fits xfrequency varied land including an oscillator tank' circuit; fmeans 'for varying fthe .resonant irequency' lof the tank -circuit, fsa'idmeans including a pai'rof electron'con trolA tubes .having ftheir :anodes -connectediin push-A pull relation .across Avthe tank rcircuitgfa iirst net-l work :comprising a resistancekand a ,capacitance connectedA in series `across at Fleast fa portion nfl said tank circuit, and a second network icon-r- -nected sbetweensthe .junction .o'f rsaid .resistance and capacitance an the cathodes of said control tubes,f:said second network .comprising /a resistnance fand' capacitance in Eparallel, said k:second fretsistance fand capacitance having the :same electrical values as :said first-mentioned resistance and capacitance respectively, Iltlne `resistance of .eachoftsaid resistance elements being substan- -tially -fequal .*tothe freactance :of each :of Isaid -rcapacitance :elements at Vthe operating .v-frequency'.

Inr -anelectrical `system `:for forming' 4frelquency-:controlled oscillations, means for `gener- .ating' yself-'sustained oscillations :comprising ra ,space .discharge device and an associated reso'- nant circuit 'including' reactive elements, va pair of space .discharge devices foroontrlling the `frequencyoftsaid oscillations, said devices Thaving s an 'input andan `output. circuit, means l'for coupling the output circuit to one of the reactive :elements ovfthe resonant circuit, fa :network Acom'- prising a resistor and Va :capacitzfirv 'connected `in .series across saidoutput circuit, the resistance :of said :resistor "being 'substantially equal" to the vreactance. of Jlsaid capacitor at the-operating A4frequency, a connection for radio lfrequency'between the input circuits of said controldevices andthe .junction :between `said resistor Yand capacitor, and asecond @network connected between said junc tion and a common vterminal of --said Ydischarge devices.

'5I-n combination, -an electrontube oscillator capable "of thaving -its frequency varied 'and including 4an oscillator *tank Vcircuit, meansfior varying the resonant*lfreque'ncyl or fthe tank jc'ircuit,-said means including' a *pair of' electron coniy trol .mbos-feaowhoving" at ioastf o -oathooja anode and a'fgri-d, `said anodes being connected to said tank circuit, a series vresistance-capacitan'ce circuit connec'te'ol y'across at least la portiongof the tank eircom reach' of s aidigrios being connected to acenter vtap of Vsaid'last-named circuit, a parallel resistance-capacitance circuit :between said grids'and avpoirit 'of v low yradio frequency poten# tial, fand r-rn'eans y'for applyinga 'control voltage to said l'grids, the d'elements uoff .said resistance-capaci; tance circuits allhaving the `same yvalue o'f impedance'attheoperating frequency.

f5. In 'corribination, an electron tube oscillator capable of -'having its frequency varied `and comprising an oscillatortank 4`ci'rcuit"including an indc'tan'ce, lmeans vfor varying `the 'resonant frequency of fthe tank v,circuit including" 'a pair of lectron Icontrol tubesyeach having at least a cathode,l fan anode, iand a grid, two seriesresistor-condenser 'fcircufits connected .across V'at least a portion orsaid tank cirouitmduoto'noe, the 'resistor o'o'ne ser" K circuit"beingy :connected tothe sameffs'ide of the ankinductace asthe con.

l y Y I, gasmcircuit a v'connection fror'n the grid -oij' one of the control tubes ,to `the junction r'etwee'nftlije resistor anol V condenser of one sor1es circuit asimilar Ycom;o o-tion ,between the' grid of 'thefothu r tubeandtnejjunction'in the second;series'circuit, connections between each of said i 1 1'I'rct `i `c i' isjarid groundffor radio frequenf cies including"parallel'resistor-.condenser circuits, tliefresisto'r'jand condenser of each 'parallel circu'it*k eing` respectively equal 'in impedance .atthe operating irequency to the izesis'torand [condenser -oi the Yseries circuit with, which Vit .isas-` sociated,jand meansforapplying' -control ,voltage' fto said 'grids' to controlfthe operation of the controitubes.

claini 5-inwhih Vthe'-'condenser in each parallel be? described circuit includes theftubeg'ridz-'tofcathode capac- .i .l I Y, s .I tiff,

7. -Iny combination; an-'electronv tube oscillator capable 'oi' having its frequency varied-'andimcludingan oscillator tank circuit, means for varying the resonant frequency -of thetank -circuit, saidmeans including a pair. of electron control tubes each having at -leastI a catliode',.an anode, and a control grid, saidanodes being connected together and tothe.tankfcircuit,.,two'see cathodeseach including alresistor and condenser in parallel, each of said last-named resistors and condensers beingV of the same value asthe-resistor and .condenser of the seriesqcircuit associated withthesame grid. 1 y r j y 1 .8.- A. circuit for. deriving a quadrature voltagefrom a source of oscillations, comprising a series resistor and condenser connectedV across said source, and Va parallel .resistor and condenser connected between the junction of said resistor and condenser and a pointof substantially .zero potential, said resistors rbeingequal in valuesa idcondenser s being equal in value, and .the resistanceY of ,each resistor beingsube *stantially equal..to theI-reactance -of its associated conder 1se1 at :the operating f frequency, and 'said ,quadrature` voltage .appearing at said junc- OI1.. f v gj In a frequency-control systemfor-an oscillator in whionthe output .of the control .tube means is connected in the oscillatorcircuit, a source of quadrature voltage `for the control tube means comprising a resistor and a reactive impedance in series across the oscillator .out/put, a resistor and a reactive impedance connected -in parallel between .the-.juncture of said Vilrst `resistor and said first reactive impedance and a point of substantially zero alternating potential, a connectionlbetween said juncture landthe grid means of -svaid control tube means, said resistors being equal and said `reactive impedances .being equaland of the same'sign, and the resistance of each resistor being substantially equal to the associated reactive impedance at the operating frequency, rand means for applying control voltage to ,'saidgrid means Y .l 1'0. In a ir'equency. control system for an oscil-. lat'or"j in which: the l output circuit of a control tube controls the, frequency `of the oscillator,-a source of quadrature voltage for -the control tube comprising a series .resistor and. reacton con nected lacross atleast part of said oscillator,- and av .parallel resistor andA reactor connectedY between the juncture of 'said mst-named resistorl and reactor 'and' a point or substantially zero alternatingpotential 4in said `oscillator circuit, said second'namedresistor being equalin valuev to said first named resistor, and said secondnamed. reactor'being Vequal in` value and of the. same sign as s aid rst named reactor, and the res'istance` of each. resistor being substantiallyequal to the reactanceof the..associatedreactor atthe operating frequency.

v11. In combination, ,an oscillator ci cuitzhavatleast one winding .aliiiltgrmediate point,

ing a resistoranda condenser connected in series across apart ofrsaid windingfsaid part including Iportions onk eitherssideof said. intermediate `point, andi-a resistors'and condenserLconnected 1in lparallel `in; an alternating current pathex'- tending betweens'aid `point ofixed .potential and -the junction-'of theirstnamedresistor andcon'- denser; afrequency control' tube. having an? out- 'put circuit coupledtofsaidoscillator circuit,Y and 'a connection between a controlaeleinent -control v'.tube" and said junction.: a .15

12. The-combination claimed in claim 11', characterized in .that said winding is connected in the anode circuit of said oscillator. `13.'The combination claimed in` claim 411,

.characterized in that the impedances of said resistors and condensers are substantially equal in; ...magnitudev vat .the :mean operatingfrequency 'oftthe oscillatory.: i

.f14.A AI frequencyI control :system for an oscillatorihavingaltankfcoih comprising a pair of control tubes having. their-respective plates connected to electrically `spaced rpoints on said tank coil, and sources .of'o'pposite phase quadrature voltagesl for said control tubes respectively, each of said sources consisting of la'series condenser and resistor connectedacross part of said tank 1 coil and a..parallel resistor vand condenser connectedbetween 4the juncture of said series resistor` and condenser ainda-.point of substantially zero alternating potential, both' of` the resistors and both of the condensers being equal invalue,

' and a connection between each'of said junctures and the associated control-tube.

15. A circuit for controlling the frequency of an oscillator having a tuned tank coil, comprising a v-pairfo'f control' tubes; the -plates 7 of said control tubes being 'connected together and to the tank coil, means for -providingquadrature voltages to the' grids of "said control tubes, said quadrature voltages being of oppositefphas'e with respect to each other, saidrn'eans consisting o f a series resistor-condenser circuit connected between points "of -different potential on said tank coil, and'la resistor and-condenser of the same valueconnected in 'parallel between the juncture of said first lresistor and condenser and a point of substantially zero alternatingvoltage, a confrequency is variable, 'aV pair ofcontrol tubes,"

nection between said juncture and the'grid of:

one ofthe control tub'esfand la source of separate control ,voltage foreach of vsaid tube'grids.

16.'In 4combinationv with an oscillator whose each having'an output! electrode! and a-'control electrode; means -forconnecting said output elec-l4 trodesA toizthe oscillator to applythe outputvolt#- age of said tubes to the oscillator,'r'neans for derivinga quadraturevoltage from the oscillator and forI applying saidy voltage tto the control electrodes of said .tubea'and fmeans for applying control voltage-to the control electrodes of said tubes, the voltages applied by two-ofthe afore-V said meansb'einga-in oppositewphase, vand the voltageapplied by the other of-` said means ibeing.- in the same phase; said second-means comprisingl resistance andreactance elementsfoithe series path, said network :beingconstructed f and J arof said potential, said resistance elements being equal 'in value, said reactance elements being equal in value, and each resistance being substantially equal to the associated reactance at the operating frequency, and said quadrature voltage appearing at said junction- YVONNE TRAVIS. Administratri of the Estate of Charles Travis,

Deceased. 

